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Sound Waves and Room Acoustics Essay

We generally think of the speakers in our stereo or home theater systems as the final link in the audio chain — and the one that makes the biggest difference to our ears. But there’s much more to the sound we hear than just where you place your speakers in a stereo or home theater setup, and what comes out of them. You might not even realize it, but your room plays a rather large part in the sound that you hear from your system. And as with any other component, there are steps you can take to improve your room’s performance.

Sound energy is in the form of invisible waves. Since our hearing can perceive sounds from 20-20,000 Hz, we’re talking about wavelengths that range from 11/16″ at 20,000 Hz, to over 56 feet at 20 Hz.

Why your room matters The sound that you hear in any room is a combination of the direct sound that travels straight from your speakers to your ears, and the indirect reflected sound — the sound from your speakers that bounces off the walls, floor, ceiling, and furniture before it reaches your ears. Reflected sounds can be both good and bad. The good part is that they make music and movie dialogue sound much fuller and louder than they would otherwise. If you’ve ever played your speakers outdoors where there are no walls to add reflections, you’ve probably noticed that they don’t sound as good — thin and dull, with very little bass.

Reflected sound can add a pleasant spaciousness to your sound. In addition to the sound from your speakers, you hear reflected sound from your room’s four walls (above left). Your room’s ceiling and floor contribute reflected sound, as well (above right). The bad part is that these same reflections can also distort sound in a room by making certain notes sound louder while canceling out others. The result may be midrange and treble that’s too bright and harsh or echoey, or bass notes that are boomy, with a muddy “one-note” quality that drowns out deep bass. Because these reflections arrive at your ears at different times than the sound from your speakers, the three-dimensional “soundstage” created by your speakers and the images of the instruments and singers may become vague or smeared. Applying the 1 : 1.4 : 1.9 room dimension ratio to a room with an 8-ft. ceiling yields dimensions of 8’H x 11.2’W x 15.2’L.

Basic tips on taming your room’s reflections

Reflections: they are echo-like sounds that reflect off walls and reach your ears slightly after the original sound, the ear however –and the whole auditory system, which includes the brain- detects this as part of the original sound. Therefore, with many echoes the sound system gets mixed up and we cannot somehow identify the original sound. Take a movie for example, if we watch a movie at home and there are many reflections therefore many echoes, we will not enjoy the movie! As you can see, when it comes to sound there are two factors: loudness and length of delay. If the reflection is too loud, or if the delay between the original sound and the reflection lasts too long, you’ll generally hear a distinct echo. That’s why it’s much more difficult to locate the source of a sound in a highly reflective room with uncontrolled echoes, or outdoors in an open field, where the only reflective surface is the ground.

Standing waves and room resonance modes Any time you have a pair of parallel reflective surfaces (like room walls, or the floor and ceiling), you’re going to experience some degree of a phenomenon known as “standing waves.” Standing waves distort the bass and lower midrange frequencies from 300 Hz on down. Standing waves are created when sound is reflected back and forth between any two parallel surfaces in your room. They affect frequencies below 300 Hz.

A room’s primary or “axial” resonance modes are based on the room’s three main axes: length, width, and height. These resonance modes create bass peaks and dips of up to 10 dB throughout the room — so the volume may sound twice as loud in some areas as opposed to others.

Absorption: The sound produced by your speakers, as well as its reflections from your room’s walls, ceiling, floor and furnishings, is actually sound energy, or acoustical energy. These sound waves cause air particles to vibrate, and when they vibrate against our eardrums, we hear sound. A basic law of physics states that energy can neither be created nor destroyed, but can be converted into another form. If it’s impossible to simply destroy all these unwanted sound reflections, how can we control them? This is where the concept of sound absorption enters the picture. If you’ve ever been inside a recording studio, radio or TV station, concert hall, or music practice room at a school or music store, you’ve probably seen some type of sound-absorbing material. Applying absorptive material to walls and other reflective surfaces is the primary method for taming unwanted reflections. Dense, porous materials like polyurethane foam and fiberglass tend to be the most popular choices.

These materials absorb sound by converting the acoustical energy into heat. This happens when the air particles are driven into motion by the sound waves, then attempt to pass through the dense sound-absorbing material, resulting in a very small amount of heat-generating friction. The sound-absorbing effectiveness of some common room surfaces. Fibrous materials like carpet and drapes provide significant absorption above 500 Hz, but have little effect on lower frequencies. Conversely, window glass and drywall can absorb bass frequencies, but are very reflective above 500 Hz. The most successful approaches combine materials like these with professionally-designed room treatment products.

Diffusion: Diffusion products can be used to treat many of the same problems that absorption is used for. Again, diffusion will rid your room of echoey reflections without replacing them with acoustic deadness.

Treating your room with absorptive materials can get you most of the way there, but if your movies and music still don’t sound quite right, then diffusion is another option. Diffusion is the scattering or redistribution of acoustical energy. The advantage of diffusion is that because the sound energy is scattered rather than absorbed, that energy isn’t lost, thereby maintaining more of a “live” sound in your room. It’s difficult to describe this type of effect because it’s completely rooted in advanced mathematics. Concert halls, recording studios, and broadcast facilities that use diffusion rely on on professional products based on mathematical number theory. And though diffusive materials are more difficult to implement in a home listening room than absorptive materials, it is possible. If you’re using conventional (non-dipole) surround speakers in a home theater system, you can achieve much of the diffuse sound of dipole speakers by treating your rear wall with diffusors and aiming your surrounds at them.

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